The present apparatus relates to energy collecting means, and more particularly to an improved solar energy collector which concentrates, absorbs and transfers heat to a fluid.
The practical, efficient utilization of solar energy has for generations been the object of much effort and study. In recent times, as the hazards of air pollution due to the use of fossil fuels have become apparent, along with the potential hazards of nuclear energy sources, considerable attention has been focused upon devising economical and practical means for collecting energy of solar rays. Even more lately, shortages of petroleum products, along with the rapidly rising cost thereof, has lent new urgency to the search for practical means for making use of solar energy.
To date, although much effort has been expended in the development of prototype and experimental solar energy collecting and storage apparatus, none have attained any degree of practical, economic success. The expense and/or inefficienty of construction of any apparatus so far devised has inhibited the development of practical solar energy collection means. Ordinarily, solar collectors comprise a metallic plate or the like enclosed by a glass cover, and solar rays heat the metal therebeneath. Energy from the heated metal is then collected by a heat transfer fluid, such as air or water which is passed beneath or over the metal whereby it is heated. The heated fluid is then stored untl needed, then pumped through other heat transfer devices, e.g., radiators, which extract heat energy from the fluid.
The principal difficulty with prior art solar energy collection devices has been convective loss. This important heat loss factor in solar collectors stems from the transfer of heat by moving air. If high temperature solar collector operation is to become practical, convective energy loss must be minimized. Previous attempts to control convection, in the region between the absorber plate and the glass sheet or between sheet layers, involve: (1) evacuation, (2) use of a honeycomb structure, (3) use of a high molecular weight inert gas, or (4) limitation of the temperature difference between the absorber plate and the sheet.
Each of these attempts is very costly and is plagued with serious difficulties. (1) Some of the difficulties with evacuated collectors are: (a) they are prone to spectacular implosions induced by vandalism, hail, etc.; (b) they must not collapse when subjected to atmospheric pressure; (c) the enclosure and sheet geometry, necessary to resist atmospheric pressure, results in high reflective losses; (d) it is very difficult to maintain a high vacuum over an extended time period; (e) as the temperature difference increases or as the linear dimension of the evacuated region is made larger, a higher and higher degree of evacuation is necessary to prevent reestablishement of convective flow. (2) Some disadvantages of honeycomb structures are: (a) they are fragile and costly; (b) they absorb solar energy before it can reach the black absorbing element; (c) if they become dirty, there will be a large increase in the proportion of solar energy wasted by absorption in the honeycombs; (d) if the temperature differential across the honeycomb is not sufficiently small or if the sheet spacing is large, convective flow can be reestablished despite the honeycomb; (e) if the collector heat transfer fluid flow should stop, honeycombs have been known to melt, necessitating costly and time consuming repairs. (3) Some difficulties with an inert high molecular weight gas are: (a) it must be contained without loss, and without air seeping in, for many years; (b) the gas is more difficult to obtain than air; (c) if the sheet spacing is large, the use of such a gas gives essentially no advantage over air; (d) if the temperature difference is not small, convective flow will be fully established despite the inert gas. (4) Attempting to limit the temperature difference between the absorber plate and the sheet has the difficulty that it is not always possible to carry out, e.g., when the ambient temperature is low (e.g., on cold days) or if the required heat transfer fluid temperature must be high (e.g., for absorption air conditioning). Hence, this method can not reduce convection by an appreciable amount.
Because of there difficulties with controlling convection, many suppliers of solar collection apparatus have chosen to use apparatus prone to convective loss.
The maximum temperature of such apparatus is limited to relatively low values. Typically such values have been below 150.degree. F. Accordingly, the stored heat transfer fluid can be maintained at no more than 150.degree. F. or thereabouts and usually much less.
A still further difficulty has been that the sun's orientation with respect to some collectors is at an optimum for only one or two hours in a ten-hour day. Accordingly, for a ten-hour period of sunlight, a large percentage of the available solar energy may not be used.
In order to overcome this problem, practitioners have devised various types of reflectors for use in conjunction with heat absorbers. Typically, the reflectors are pivoted or swiveled so as to maintain the sun's rays focused upon the absorptive apparatus. Said reflectors are commonly surfaces of rotation such as sections of spheres or paraboloids. However, the cost and complexity of these movable apparatus, hereinafter termed "steerable reflectors", is often prohibitive. Further, like any complex movable object, they are susceptible to wear and breakage so that they decrease the overall reliability of the heating system. Further, additional energy is required to drive these steerable reflectors. Accordingly, it will be appreciated that it would be highly desirable to provide an improved absorptive apparatus which absorbs solar energy with relatively little loss and does not require diurnal tracking of the sun. This is not possible with conventional focusing techniques. Also, the efficiency of the apparatus in absorbing diffuse light is much greater than that of focusing collectors.